The use of ultrasound for medical diagnostics and therapy is well known. Diagnostic techniques are based on the production and transmission of ultrasound waves into the body, and detection of the scattered echoes from the scanned region. Therapeutic methods are generally based on the use of focused beams of ultrasonic energy to produce high-powered mechanical energy for disintegration of medical targets by heat and ablation or cavitation caused by pressure waves. In body fluids such as blood or in the intercellular fluids in living tissue, the application of ultrasonic energy often leads to the creation of bubbles which grow in volume by a process known as rectified diffusion and eventually implode releasing large amounts of energy and generating sites of locally high temperature and pressure for very short periods of time.
In industry cavitational effects are used for a wide variety of applications from cleaning of objects to initiating chemical reactions.
Conventional ultrasound signals are generated by transducers powered by a sinusoidal waveform such as that shown in FIG. 1. The horizontal axis represents time measured in μsec and the vertical axis the voltage applied to the transducer. When the applied electrical signal is in the frequency range of the transducer's frequency bandwidth and the signal is at least a few cycles long, the pressure wave generated by the transducer is of similar shape. The waves that are emitted from the transducer travel through the media as longitudinal waves (the transverse waves usually attenuate very rapidly in tissue and thus are ignored herein) having alternating compression and de-compression regions corresponding to the positive and negative portions of the waveform shown in FIG. 1. When the wave passes through a fluid, gases trapped inside dust motes or other particles in the fluid, or on the walls of the region containing the fluid will be drawn out from the fluid forming a small bubble. If the acoustic power density is small, then the bubble will oscillate around a relatively constant radius. This process is known as stable cavitation. If the power density is high, then gas diffuses into the bubble during the de-compression half-cycle of the sound waves and diffuses out from the bubble during the compression half-cycle. The rate of diffusion is proportional to the radius of the bubble and therefore the rate of diffusion into the bubble (which occurs when the bubble has expanded during the de-compression phase) exceeds that of the rate of diffusion out of the bubble (which occurs when the bubble has been compressed). The net result is that the radius of the bubble increases as the bubble oscillates. This process is known as rectified diffusion. Once the bubble's radius reaches a critical value, which depends on the power and frequency of the ultrasonic energy, it can no longer remain stable and the pressure caused by the next compression half-cycle will cause the bubble to implode, i.e. the fluids in the vicinity of the bubble oscillate with such an amplitude that the bubble breaks into small fractions.
In medical applications the energy released by the implosion of the bubbles in the rectified diffusion process is used to destroy near by cells. Various methods are known to produce cavitation at the desired location. For example, U.S. Pat. No. 5,219,401 teaches the use of relatively low power ultrasound energy to produce stable cavitation resulting in a population of bubbles at a site and then applying a second signal at another frequency and higher power to cause the bubbles to implode. U.S. Pat. No. 6,413,216 teaches the use of an unfocused transducer operating at a low frequency to create bubbles in a treatment area of a patient followed by the use of a focused ultrasound beam at a different frequency aimed at a specific region within the treatment area in order to cause cavitation and thereby create a lesion at a desired location. U.S. Pat. No. 5,827,204 teaches a method reported to produce large vaporous cavitation bubbles in a small confined area. The method comprises generating a low frequency signal having amplitude less than the cavitation threshold to produce a population of bubbles and superimposing on this signal a high frequency signal. The amplitude of the resulting modulated signal exceeds the cavitation threshold at the focus of the modulated beam. The aim of the art is to increase the magnitude of the cavitational effect while at the same time carefully controlling the region in which cavitation takes place in order to allow more precise therapeutic treatment while preventing unintended damage to surrounding cells
In recent years it has been shown that sonochemically active cavitation can be enhanced an order of magnitude by superimposing the second harmonic onto the fundamental in insonation [S. Umemara, K. Kawabata, and K. Saski: “Enhancement of Sonodynamic Tissue Damage Production by Second-Harmonic Superimposition: Theoretical Analysis of its Mechanism”, IEEE Transactions on Ultrasonics, Ferroelectricss and Frequency Control, 43 (1996) 1054-1062]; and [S. Umemara, K. Kawabata, and K. Saski: “In vitro and in vivo enhancement of sonodynamically active cavitation by second-harmonic superimposition” J. Acoust. Soc. Am. 101 (1997) 569-577.] In another study it has been shown that combined irradiation with two or more orthogonal beams, of different ultrasound frequencies focused at a common location produces a significant increase in cavitation effects over single frequency irradiation. [Ruo Feng, Yiyun Zhao, Changping Zhu, T. J. Mason: “Enhancement of ultrasonic cavitation yield by multi-frequency sonification”, Ultrasonics Soinochemistry 9 (2002) 231-236.]
It is a purpose of the present invention to provide an apparatus and method for providing focused ultrasonic waves having a waveform at the focal point that is modified to cause enhanced cavitation.
Further purposes and advantages of this invention will appear as the description proceeds.